Add CHANGELOG entries for #1113, #1116, #1117, #1118, #1119, #1120

[petergoldstein]

Summary

• Adds Performance entries for ResponseProcessor#value_from_tokens: parse integer in place #1113 (ResponseProcessor#value_from_tokens in-place integer parsing), ResponseBuffer#process_single_getk_response avoid one allocation #1117 (ResponseBuffer getk allocation reduction), RequestFormatter: use frozen string literals #1118 (frozen string literals in RequestFormatter), and Refactor KeyRegularizer to reduce allocations #1120 (KeyRegularizer and multi-key path refactor)
• Adds Bug Fix entry for Fix and add tests for ResponseBuffer compaction #1119 (restores and corrects ResponseBuffer compaction logic broken by Get rid of unused COMPACT_THRESHOLD constant #1116)
• Adds Maintenance entry for Get rid of unused COMPACT_THRESHOLD constant #1116 (removal of COMPACT_THRESHOLD, subsequently corrected by Fix and add tests for ResponseBuffer compaction #1119)

All six PRs were merged to main but had no CHANGELOG entries.

Test plan

• Verify CHANGELOG entries accurately describe the linked PRs
• Confirm no duplicate entries exist

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